This invention relates to the general field or turbogenerator controls and more particularly to an improved high speed turbogenerator control system having variable frequency output power which provides electrical power to motors which have power requirements that normally vary in a repetitive manner over time.
There are many industrial and commercial applications that utilize electrical motors to produce repetitive axial motions. The electrical motor""s rotary motion can be converted into axial motion by any number of mechanisms such as cams, cranks, scotch yokes, or cable drums just to name a few. In any such application, the electrical power requirement of the motor is inherently variable and is cyclically locked to the repetitive axial motion. The motor power in these applications varies both due to inertial effects (the need to accelerate and decelerate the axially moving components of the system and the need to accelerate and decelerate the rotationally moving components of the system) and due to the work effects (changes in the work performed by the axially moving components as a function of their axial position and velocity). The magnitude of the motor power variation with time can be many times the average power requirement of the motor. Both the inertial effects and the work effects can cause the motor to function as a generator which produces electrical power at various times in the system""s cyclical motion.
An elevator is one well-known example of an electrical motor producing axial motion wherein the motor""s electrical power requirements vary with the passenger load, the axial velocity of the elevator and the axial acceleration/deceleration of the elevator. Deliberate deceleration or braking can be achieved by recovering the excess energy in the elevator""s mechanical system (e.g. during the descent of a heavily loaded elevator) utilizing regeneration to convert that mechanical energy into electrical energy which can go back into an electrical distribution system.
A less well known example of a motor producing repetitive axial motion is a pump-jack type oil well. Also known as a walking beam (a large beam arranged in teeter totter fashion) or a walking-horse oil well, the pump-jack oil well generally comprises a walking beam suitably journaled and supported in an overhanging relationship to the oil well borehole so that a string of rods (as long as two miles) can be attached to the reciprocating end of the walking beam with the other end attached to a lift pump chamber at the bottom of the bore hole. A suitable driving means, such as an electrical motor or internal combustion engine, is connected to a speed reduction unit which drives a crank which in turn is interconnected to the other end of the walking beam by a pitman.
Typically, pump-jack oil wells utilize an induction motor powered by constant frequency, three-phase electrical power from a utility grid. The pump-jack pumping cycle varies the induction motor""s speed only slightly as allowed by plus or minus a few percent of motor slip. However, the induction motor power typically varies over the pumping cycle by about four (4) times the average motor power level. At two (2) points in the pumping cycle, the motor power requirement peaks and at two (2) other points, the motor power requirements are at a minimum. Typically, at one of these minimum power requirement points in the pumping cycle, the induction motor extracts enough kinetic energy and/or work from the moving masses of the well to be able to function as a generator and produce electrical power which must be absorbed by the utility grid.
Whether the pump-jack oil well is driven by an induction motor or by an internal combustion engine, there is excess mechanical energy at some point(s) in the pumping cycle which must be absorbed to prevent excessive velocity induced stresses in the pump-jack oil well moving parts. When a pump-jack oil well is powered by an internal combustion engine, engine compression is the means by which this energy is dissipated (compression losses) while in the normal utility grid powered induction motor system, the induction motor is periodically driven at overspeed causing it to return power to the utility grid.
A micro turbogenerator with a shaft mounted permanent magnet motor/generator can be utilized to provide electrical power for a wide range of utility, commercial and industrial applications. While an individual permanent magnet turbogenerator may only generate 24 to 50 kilowatts, powerplants of up to 500 kilowatts or greater are possible by linking numerous permanent magnet turbogenerators together. Peak load shaving power, grid parallel power, standby power, and remote location (stand-alone) power are just some of the potential applications for which these lightweight, low noise, low cost, environmentally friendly, and thermally efficient units can be useful.
The conventional power control system for a turbogenerator produces constant frequency, three-phase electrical power that closely approximates the electrical power produced by utility grids. If a turbogenerator with a conventional system for controlling its power generation were utilized to power a pump-jack type oil well, the turbogenerator""s power capability would have to be sufficient to supply the well""s peak power requirements, that is, about four (4) times the well""s average power requirement. In other words, the turbogenerator would have to be about four (4) times as large, four (4) times as heavy, and four (4) times as expensive as a turbogenerator that only had to provide the average power required by the oil well rather than the well""s peak power requirements.
There are other inherent difficulties present if a turbogenerator with a conventional power control system is used to provide electrical power for a pump-jack type of oil well. If, for example, the oil well is in the part of the pumping cycle where it normally generates rather than consumes power, the operating speed of the rotating elements of the turbogenerator will tend to increase. The fuel control system of the power control system will attempt to reduce the fuel flow to the tubogenerator combustor in order to prevent the turbogenerator""s rotating elements from overspeeding which, in turn, risks quenching the flame in the combustor (flame out). A minimum fuel flow into the combustor must be maintained to avoid flame out. This results in a minimum level of power generation, which together with the power produced by the oil well itself, must be deliberately dissipated as wasted power by the turbogenerator system, usually with a load resistor but sometimes with a pneumatic load, either of which will reduce the turbogenerator system efficiency.
Also, when the power requirements for the oil well fall below the well""s peak requirement, the conventional turbogenerator control system will reduce the turbogenerator speed and the turbogenerator combustion temperature. Since the present systems do not have any means to dissipate excess power, the rapidly fluctuating load levels and unloading operation produce undesirable centrifugal and thermal cycles stresses in many components of the turbogenerator system which will tend to reduce turbogenerator life, reliability and system efficiency.
When a pump-jack type oil well is powered by constant frequency electrical power from a utility grid on a conventionally controlled turbogenerator, the oil extraction pumping rate may not be sufficient to keep up with the rate at which oil seeps into the well. In this case, potential oil production and revenues may be lost. Alternately, the oil extraction pumping rate may be greater than the rate at which oil seeps into the well. In this case, the oil well may waste power when no oil is being pumped or it may be necessary to shut down the oil well for a period of time to allow more oil to seep into the well.
For the reasons stated above, the conventional turbogenerator control system is not generally suitable for pump-jack oil well systems.
The turbogenerator control system of the present invention includes a high frequency inverter synchronously connected to the permanent magnet motor/generator of a turbogenerator, a low frequency load inverter connected to the induction motor(s) of the pump-jack oil well(s), a direct current bus electrically connecting the two (2) inverters, and a central processing unit which controls the frequency and voltage/current of each of the inverters. This control system can readily start the turbogenerator.
Alternately, a turbogenerator control system, when utilized to generate power, can include a bridge rectifier which converts the high frequency three-phase electrical power produced by the permanent magnet motor/generator of the turbogenerator into direct current power, a low frequency load inverter connected to the induction motor(s) of the pump-jack oil well(s), a direct current bus electrically connecting the rectifier to the low frequency load inverter and a central processing unit which controls the frequency and voltage/current of the low frequency load inverter. The configuration of this control system can be modified by switching electrical contactors or relays to allow the low frequency load inverter to be used to start the turbogenerator.
Throughout the oil well""s pumping cycle, the central processing unit increases or decreases the frequency of the low frequency load inverter in order to axially accelerate and decelerate the masses of the down hole steel pump rod(s) and oil and to rotationally accelerate and decelerate the masses of the motor rotor and counter balance weights.
Precisely controlling the acceleration and deceleration of both the axially moving and rotational moving masses of the oil well allows relatively independent control of the rate at which shaft power and electrical power can be converted into kinetic energy. This kinetic energy can be cyclically stored by and extracted from the moving masses. Just as changing the rotational velocity versus time profile of the well""s rotating components allows the well to function as a conventional flywheel, changing the normal axial velocity versus time profile of the well""s massive down hole moving components and oil, allows the well to function as an axial flywheel. Adjusting the frequency of the low frequency load inverter and the resulting speed of the well""s induction motor also allows the oil pumping power to be controlled as a function of time. The sum of the well""s oil pumping power requirements and the power converted into or extracted from the kinetic energies of the moving oil well masses is controlled so as to be nearly constant.
Thus, the combination of tailoring oil well pumping power as a function of time and precisely controlling the insertion and extraction of kinetic energy into and out of the moving masses of oil wells results in stabilizing the power requirements demanded of a turbogenerator powering pump-jack oil wells. This is turn allows the size of the turbogenerator to be down sized by a factor of perhaps four to one (4 to 1), avoids extreme variations in turbogenerator operating speed and combustion temperature as well as avoids possible damage to the turbogenerator caused by cyclical variations in thermal and centrifugal stresses and possible damage to the controller/inverter electronics caused by variation in turbogenerator voltage.
It is, therefore, a principal aspect of the present invention to provide a system to control the operation of a turbogenerator and its electronic inverters.
It is another aspect of the present invention to control the flow of fuel into the turbogenerator combustor.
It is another aspect of the present invention to control the temperature of the combustion process in the turbogenerator combustor and the resulting turbine inlet and turbine exhaust temperatures.
It is another aspect of the present invention to control the rotational speed of the turbogenerator rotor upon which the centrifugal compressor wheel, the turbine wheel, the motor/generator, and the bearings are mounted.
It is another aspect of the present invention to control the torque produced by the turbogenerator power head (turbine and compressor mounted and supported by bearings on a common shaft) and delivered to the motor/generator of the turbogenerator.
It is another aspect of the present invention to control the shaft power produced by the turbogenerator power head and delivered to the motor/generator of the turbogenerator.
It is another aspect of the present invention to control the electrical power produced by the motor/generator of the turbogenerator.
It is another aspect of the present invention to control the operations of the high frequency inverter which inserts/extracts power into/from the motor/generator of the turbogenerator and produces electrical power for the direct current bus of the turbogenerator controller.
It is another aspect of the present invention to control the operations of the low frequency load inverter which uses power from the direct current bus of the turbogenerator controller to generate low frequency, three-phase power.
It is another aspect of the present invention to minimize variations in the fuel flow rate into the turbogenerator combustor over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to minimize variations in the combustion and turbine temperatures of the turbogenerator over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to minimize variations in the operating speed of the turbogenerator over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to minimize variations in the shaft torque generated by the turbogenerator power head and delivered to the motor/generator of the turbogenerator over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to minimize variations in the shaft power generated by the turbogenerator power head and delivered to the motor/generator of the turbogenerator over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to minimize variations in the level of electrical power extracted from the motor/generator of the turbogenerator and converted into direct current power by the high frequency inverter, or the bridge rectifier, over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to minimize variations in the level of electrical power extracted from the direct current bus and converted into low frequency, three-phase power by the low frequency load inverter over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to minimize variations in the level of electrical power delivered to, and utilized by, the induction motor(s) of the pump-jack oil well(s) over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to provide a control system that sets the average frequency of the low frequency load inverter over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to provide a control system where the average frequency of the low frequency load inverter over the operating cycle of a pump-jack oil well can be set so that the oil pumping rate of the well is matched to the rate at which oil seeps into the well from the surrounding oil ladened matrix. Thus, the well neither runs dry nor has to produce oil at less than the well""s capacity.
It is another aspect of the present invention to provide a control system that varies the instantaneous frequency of the low frequency load inverter over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to provide a control system that varies the instantaneous voltage or current of the low frequency load inverter over the operating cycle of a pump-jack oil well.
It is another aspect of the present invention to provide a control system where the variation in the instantaneous frequency of the low frequency load inverter over the operating cycle of a pump-jack oil well is the primary means by which the system reduces the variations in power required by the induction motor of the pump-jack oil well.
It is another aspect of the present invention to provide a control system where the variation in the voltage or current of the low frequency load inverter over the operating cycle of a pump-jack oil well is the secondary means by which the system reduces the variations in power required by the induction motor of the pump-jack oil well and simultaneously is the primary means by which the system controls the slip and maximizes the efficiency of the inductor motor.
It is another aspect of the present invention to provide a control system with that can precisely control the insertion of kinetic energy into, and the extraction of kinetic energy from, the moving masses of the pump-jack oil well over the operating cycle of the well.
It is another aspect of the present invention to provide a control system that allows the rotational moving masses of the pump-jack oil well to function as a flywheel for energy storage.
It is another aspect of the present invention to provide a control system that allows the axially moving masses of the pump-jack oil well to function as an axial flywheel for energy storage.
It is another aspect of the present invention to provide a control system that can precisely control the instantaneous pumping work being performed by a pump-jack oil well or the instantaneous pumping work being extracted from a pump-jack oil well over the operating cycle of that well.
It is another aspect of the present invention to provide a control system that causes the total of the instantaneous pumping energy required/produced by pump-jack oil well(s) and the instantaneous kinetic energy extracted/inserted from/into pump-jack oil well(s) to be nearly constant over the operating cycle of the well(s).
It is another aspect of the present invention to provide a control system that utilizes the phase relationship of the pump-jack oil well induction motor voltage and current to both measure the resonant velocities of the down hole rod string and to damp these resonances with appropriate modulations in the torque of the induction motor.
It is another aspect of the present invention to provide a control system that soft clamps the maximum and minimum frequencies of the low frequency load inverter to avoid excessive rod stresses at high frequencies, to avoid oil well pumping direction reversals, and to simultaneously minimize the excitation of rod string resonances.
It is another aspect of the present invention to provide a control system that soft clamps the maximum voltage of the low frequency load inverter to avoid excessive voltage stresses on inverter and motor components while simultaneously minimizing the excitation of rod string resonances.
It is another aspect of the present invention to provide a control system that soft clamps the D.C. bus voltage for safety.
It is another aspect of the present invention to provide a control system that minimizes thermal and centrifugal stress cycle damage to the turbogenerator""s combustor, recuperator, turbine wheel, compressor wheel, and other components that can be caused by variations in turbogenerator operating power level, speed or temperature and which are, in turn, induced by the cyclical nature of pump-jack operation.
It is another aspect of the present invention to provide a control system that minimizes the risk of combustor flame out that can occur when conventional turbogenerator fuel control systems reduce combustor fuel flow when the pump-jack""s power requirements are at a minimum or are reversed during the pumping cycle.
It is another aspect of the present invention to provide a control system that avoids the need for parasitic loads with their resulting inefficiencies and avoids the inefficiencies associated with off optimum operations when fuel flow, temperature, and speed vary widely.
It is another aspect of the present invention to provide a control system that allows the peak electrical power required by a pump-jack oil well to be reduced by a factor of about four to one.
It is another aspect of the present invention to provide a control system that allows the size, weight, and cost of a turbogenerator that powers a pump-jack oil well to be reduced by a factor of about four to one.
It is another aspect of the present invention to provide a control system that allows the size, weight, and cost of the induction motor utilized by a pump-jack oil well to be reduced by a factor of about four to one.